One of the primary goals of plant genetic engineering is to obtain plants having improved characteristics or traits. These characteristics or traits include virus resistance, insect resistance, herbicide resistance, enhanced stability and improved nutritional value, to name a few. Recent advances in genetic engineering have enabled the incorporation of preselected genes into plant cells to impart the desired qualities to the plant of choice. The introduced gene, i.e., xe2x80x9ctransgene,xe2x80x9d is then expressed in the cells of the regenerated plant, so that the plant will exhibit the trait or characteristic encoded by the transgene.
To express a transgene in a plant cell, the proper regulatory signals must be present and in the proper location with respect to the transgene. These regulatory signals generally include a promoter region, a 5xe2x80x2 non-translated leader sequence and a 3xe2x80x2 polyadenylation sequence. The promoter region influences the rate at which the RNA product of the transgene, and resultant protein product of the transgene, is made. Promoter activity also can depend on the presence of several other cis-acting regulatory elements which, in conjunction with cellular factors, determine strength, specificity, and transcription initiation site (for a review, see Zawel and Reinberg, Curr. Opin. Cell Biol., 4, 488 (1992)). Strong promoters are able to direct RNA synthesis at a higher rate relative to weak promoters. Constituitive promoters direct RNA production in many or all cell types.
The cauliflower mosaic virus 35S promoter (CaMV35S) is a strong, constitutive promoter in plants (Odell et al., Nature, 313, 810 (1985); Jensen et al., Nature, 321, 669 (1986); Jefferson et al., EMBO J., 6, 3901 (1987); Kay et al., Science, 236, 1299 (1987); Sanders et al., Nucl. Acids Res., 4, 1543 (1987)). This had been shown by detecting substantial levels of reporter gene proteins or mRNAs in extracts prepared from the leaves, stems, roots and flowers of transgenic plants. As a result, the CaMV35S promoter is widely used in the field of plant genetic engineering. Although the CaMV35S promoter appears to be a strong, constitutive promoter in assays involving cell extracts, detailed histological analysis of reporter gene products detectable at the cell and tissue level shows a rather high degree of variability of expression of the gene products in tissues of plants.
CaMV is a caulimovirus, a subgroup of pararetroviruses that has icosahedral capsids and infects only dicots, although the CaMV35 S promoter is a strong promoter in monocots. Sugarcane bacilliform virus (ScBV), Commelina yellow mottle virus (CoYMV) and rice tungro bacilliform virus (RTBV) are badnaviruses, a subgroup of pararetroviruses that have bacilliform capsids and infect mainly monocots. A promoter fragment isolated from CoYMV confers a tissue-specific pattern of expression that is different than the pattern conferred by the CaMV35S promoter. Transformed tobacco plants containing the CoYMV promoter linked to the beta-glucuronidase reporter gene (xe2x80x9cGUSxe2x80x9d; uidA) showed that while the CoMYV promoter is active in all organs, beta-glucuronidase activity occurs primarily in the phloem, the phloem-associated cells, and the axial parenchyma of roots, stems, leaves, and flowers (Medberry et al., Plant Cell, 4, 185 (1992); Medberry and Olszewski, Plant J., 3, 619 (1993)). In contrast, the CaMV35S promoter is active in most cell types (Medberry et al., Plant Cell, 4, 185 (1992); Medberry and Olszewski, Plant J., 3, 619 (1993)). Moreover, the CoYMV promoter is 30% as active in tobacco suspension cells and up to 25% as active in maize suspension cells compared to a duplicated CaMV35S promoter (Medberry et al., Plant Cell, 4, 185 (1992)).
Transgenic rice containing the RTBV promoter linked to the GUS gene showed strong phloem-specific promoter activity. This was consistent with the expression of this promoter in rice protoplasts. However, the RTBV promoter showed only weak activity in maize protoplasts (Bhattacharyya-Pakasi et al., Plant J., 4, 71 (1993); Yin et al., Plant J., 7, 969 (1995)). In contrast, the corresponding CaMV promoter shows strong promoter activity in protoplasts and in almost all tissues of transgenic plants (reviewed by Hohn and Fxc3xctterer, Curr. Opin. Genet. Dev., 2, 90 (1992)).
Thus, what is needed is a highly expressed, constitutive promoter to express transgenes in fertile transgenic monocot and dicot plants.
The present invention provides an isolated and purified DNA molecule comprising a preselected DNA segment comprising a sugarcane bacilliform virus (ScBV) promoter, or a biologically active subunit thereof, that confers constitutively high levels of expression of operably linked preselected DNA segments in both monocot and dicot plants, plant tissue, plant parts or plant cells. While the nucleotide sequence of the genome of ScBV is known (Bouhida et al., J. Gen. Virol., 74, 1 (1993)), the location of a promoter for genomic length viral RNA was not apparent, even after nucleotide sequence comparisons of the ScBV genome with promoter sequences of closely related viruses, such as CoYMV and RTBV. Surprisingly, the ScBV promoter is a strong and constituitive promoter in many cell types, unlike the strong tissue specific expression observed for CoMYV and RTBV promoters. A preferred embodiment of the invention is a preselected DNA segment comprising a ScBV promoter comprising SEQ ID NO:3, i.e., a preselected DNA segment that corresponds to nucleotide positions 5999-7420 of the ScBV genome. As described herein below, the ScBV promoter confers constitutive and vascular gene expression in A. sativa and A. thaliana. Thus, the ScBV promoter can be employed for constitutive or tissue-specific plant and non-plant gene expression in monocots and dicots. For example, the ScBV promoter may be linked to genes which confer to crops resistance against pests which attack crop grasses at the stem, e.g., aphids which vector barley yellow dwarf virus, or to confer tissue-specific resistance to dicotyledonous hosts, in which organs such as the roots are targeted by pathogens, e.g., soybean cyst nematode.
As used herein, xe2x80x9cScBVxe2x80x9d includes any non-enveloped, bacilliform, DNA-containing badnavirus capable of systemically infecting Saccharum or related genera. Other distinguishing features of badnaviruses are described by Lockhart and Olszewski, in The Encyclopedia of Virology, Webster and Granoff (eds.), Academic Press, New York, N.Y. (1994)).
As used herein, the term xe2x80x9cScBV promoterxe2x80x9d means a nucleotide sequence which, when that sequence is operably linked to a preselected DNA segment that encodes a protein, RNA transcript, or mixture thereof, results in the expression of the linked preselected DNA segment, i.e., the encoded RNA and/or protein. A preferred ScBV promoter has at least about 60%, preferably at least about 80%, more preferably at least about 90%, and even more preferably at least about 95%, nucleotide sequence identity to SEQ ID NO:3, SEQ ID NO:4 or SEQ ID NO:5. Another preferred embodiment of the invention is a ScBV promoter which comprises the minimum number of contiguous nucleotides which initiate RNA transcription.
As used herein, xe2x80x9cbiologically activexe2x80x9d means that the promoter has at least about 0.1%, preferably at least about 10%, and more preferably at least about 25%, the activity of the ScBV promoter comprising SEQ ID NO:3, SEQ ID NO:4 or SEQ ID NO:5. The activity of a promoter can be determined by methods well known to the art. For example, see Medberry et al., Plant Cell, 4, 185 (1992); Medberry et al., The Plant J., 3, 619 (1993); Sambrook et al., In: Molecular Cloning: A Laboratory Manual (1989); McPherson et al., U.S. Pat. No. 5,164,316.
Further provided is an expression cassette comprising a first preselected DNA segment comprising a ScBV promoter functional in a host cell, operably linked to a second preselected DNA segment encoding a protein, RNA transcript, or a combination thereof. A preferred host cell is a plant cell, e.g., a monocot or dicot cell. Another preferred embodiment of the invention is an expression cassette comprising a ScBV promoter operably linked to a selectable marker gene. Yet another preferred embodiment of the invention is an expression cassette comprising a ScBV promoter which comprises SEQ ID NO:3.
The invention also provides methods of selecting stable genetic transformants from transformed plant cells and methods of producing fertile transgenic plants from said transformed plant cells. The method for producing transformed plant cells comprises introducing into regenemble plant cells a recombinant DNA segment which comprises a first preselected DNA segment comprising a ScBV promoter operably linked to a second preselected DNA segment so as to yield transformed cells. Then a transformed cell line is identified or selected. Exemplary transformation methods include the use of microprojectile bombardment to introduce a preselected DNA segment, encoding a phenotypically observable or detectable trait, operably linked to the ScBV promoter, into regenerable monocot plant cells. A preferred embodiment of the invention is a method whereby the expression of the recombinant DNA segment in the transformed cells imparts a phenotypic characteristic to the transformed cells, such as herbicide or antibiotic resistance.
As used herein, the term xe2x80x9crecombinant DNA segmentxe2x80x9d refers to a nucleic acid, i.e., to DNA, that has been derived or isolated from any appropriate tissue source and isolated from association with other components of the cell, such as nucleic acid or protein. The DNA may be subsequently chemically altered in vitro, so that its sequence is not naturally occurring, or corresponds to naturally occurring sequences that are not positioned as they would be positioned in a genome which has not been transformed with exogenous DNA, so that it can be sequenced, replicated, and/or expressed.
A preferred isolated recombinant DNA segment includes a first preselected DNA segment comprising a ScBV promoter functional in a plant cell operably linked to second preselected DNA segment comprising a selectable marker gene. Another preferred isolated recombinant DNA segment includes a second preselected DNA segment that corresponds to a gene that is already present in the plant genome, or one which is not normally present in the plant genome, which confers an agronomically useful phenotype to the plant, e.g., pest resistance. If the preselected DNA segment is normally present in the plant genome it may not be expressed or not highly expressed. Thus, the preselected DNA segment is introduced so as to alter the expression of the protein or RNA transcript encoded by the preselected DNA segment in the cells of the plant.
The invention also provides a method for producing a fertile transgenic plant. The method comprises introducing a recombinant DNA segment which comprises a first preselected DNA segment comprising a ScBV promoter operably linked to a second preselected DNA segment into regenerable plant cells so as to yield regenerable transformed cells. A population of transformed cells is selected or identified and a fertile transgenic plant is regenerated therefrom. The recombinant DNA segment is transmitted through a complete sexual cycle of said transgenic plant to its progeny so that it is expressed by the progeny plants. Thus, the invention also provides a transgenic plant, and seed, other plant parts, tissue, and progeny plants derived therefrom.
The transgenic plants of the invention include, but are not limited to, a transgenic T0 or R0 plant, i.e., the first plant regenerated from transformed plant cells, a transgenic T1 or R1 plant, i.e., the first generation progeny plant, and progeny plants of further generations derived therefrom which comprise and express the recombinant DNA segment. Microprojectile bombardment can be used to introduce the recombinant DNA segment into regenerable monocot cells, while Agrobacterium-mediated DNA transfer can be used to introduce the recombinant DNA into regenerable dicot cells.
Also provided is a transformed monocot or dicot plant, the cells of which comprise a recombinant DNA segment comprising a first preselected DNA segment comprising a sugarcane bacilliform virus promoter operably linked to a second preselected DNA segment The second preselected DNA segment is expressed in the transformed cells in an amount that is different than the amount in the cells of a plant in which cells only differ from the transformed cells in that the recombinant DNA segment is absent Such cells can include untransformed cells of the same part of the transformed, or transgenic, plant, in some cases. The second preselected DNA segment is expressed so as to render the transformed plant or a part thereof identifiable over the corresponding untransformed plant or part thereof.
The recombinant DNA segment is transmitted through a complete normal sexual cycle of the transformed plant to the next generation.
Also provided is a method comprising obtaining progeny from a fertile transgenic plant obtained by the method described hereinabove.
As used herein, the term xe2x80x9ctransgenicxe2x80x9d or xe2x80x9ctransformedxe2x80x9d with respect to a plant cell, plant part (including seed), plant tissue or plant means a plant cell, plant part, plant tissue or plant which comprises an isolated, purified preselected DNA segment which has been introduced into the genome of a plant cell, plant part, plant tissue or plant by a xe2x80x9cgenetic engineeringxe2x80x9d transformation method. That is, the genome of a transgenic plant cell, plant part, plant tissue or plant has been augmented by at least one preselected DNA segment. The term xe2x80x9cwild type,xe2x80x9d xe2x80x9cnative,xe2x80x9d or xe2x80x9cnontransgenicxe2x80x9d refers to an untransformed plant cell, plant part, plant tissue or plant, i.e., one where the genome has not been altered by the presence of the preselected DNA segment.
The transformation of the plants in accordance with the invention may be carried out in essentially any of the various methods available to those skilled in the art of plant molecular biology. These include, but are not limited to, microprojectile bombardment, microinjection, electroporation of protoplasts or cells comprising partial cell walls, silicon carbide fiber-mediated DNA transfer and Agrobacterium-mediated DNA transfer. Plants useful in the practice of the invention include, but are not limited to, oat, wheat, soybean, corn, tobacco, rice, barley, potato, tomato, lettuce, oilseed rape, cotton, flax, sugar beet, sorghum, sunflower, alfalfa, millet and rye.